Various types of refrigerators are applied to various appliances, such as freezers, room air conditioners, car air conditioners and others, for cooling them and for controlling their temperature. In general, vapor compression type refrigerators in which a halogen-containing refrigerants is used in a reciprocating type, turbo type and rotary type compressor are widely used in view of their ease and safety in use.
A refrigerator oil is used as a lubricating oil in vapor compression type refrigerators to prevent wear of the sliding parts of the compressor and reduce friction between them. In the pressure elevating step in which the refrigerator oil is exposed to a high-temperature and high-pressure refrigerant atmosphere, the refrigerator oil also has, in addition to the wear retarding purpose, other roles such as cooling the sliding surfaces, discharging the heat generated by vapor compression, sealing in the vapor compression stroke and removing wear particles and impurities.
Therefore, a refrigerator oil must have excellent lubricating properties such as high wear resistance and high load carrying capacity. Additionally, to achieve high thermal stability and chemical stability when used with refrigerants, insulating materials and metal parts, without having any bad influence on the parts constituting the refrigerating system. Where a refrigerator oil is used in a refrigerator, a part of it flows into a compressed refrigerant vapor and circulates through the system of the refrigerator to low-temperature parts such as capillaries or expansion valves and evaporator. Therefore, for the purpose of elevating the refrigerating effect and of improving the return of the oil from the low-temperature parts to the compressor, and for the purpose of feeding the oil to the sliding parts in low-temperature re-starting of the compressor, the refrigerator oil must have both low-temperature fluidity and compatibility with the refrigerant in the system.
Hitherto, fluoromethanes (such as trichloro-monofluoromethane (CFC-11), dichloro-difluoromethane (CFC-12) and monochloro-difluoromethane (HCFC-22)) as specified by JIS K1517, as well as fluoroethanes (such as trichlorotrifluoroethane (CFC-113), dichlorotetrafluoroethane (CFC-114)) as specified by JIS K1528 and monochloropentafluoroethane (CFC-115) have been used as halogen-containing refrigerants, either singly or in combination of two or more.
These refrigerants all have one or more chlorine atoms in the molecule and are compatible with hydrocarbon oils, though each has a different solubility. Therefore, in refrigerating system of using such refrigerants, a refrigerator oil is used as specified by JIS K2211, (1) which comprises a base oil selected from suitably refined naphthene mineral oils, paraffin mineral oils, alkylbenzenes and poly-.alpha.-olefines and mixtures thereof and antioxidant, anti-wear agent, corrosion inhibitor etc. additives and (2) which satisfies the necessary characteristics for wear resistance, load carrying capacity, heat stability and refrigerant compatibility.
In 1974, Prof. M. R. Rowland and Dr. F. S. Molina of the University of California proposed their hypothesis of destruction of the ozone layer of the stratosphere by chlorofluorocarbons. Since then, international restriction of halogen-containing refigerants, especially chlorofluorocarbons (CFC) has been planned for the purpose of protecting the environment, and investigation of substitutes for them is underway. For instance, in Japan, CFC-11, 12, 113, 114 and 115 have been designated as chlorofluorocarbons to be restricted from Jul. 1, 1989. As substitutes for them, hydrofluorocarbons such as 1,1,1,2-tetrafluoroethane (HFC-134a) , 1,1,2,2-tetrafluoroethane (HFC-134) , 1,1,1,2-tetrafluoro-2-chloroethane (HCFC-124) , pentafluoroethane (HFC-125) , 1,1-difluoro-1-chloroethane (HFC-142b), 1,1,1-trifluoroethane (HFC-143a) and 1,1-difluoroethane (HFC-152a) have been considered. As a substitute for the important refrigerant CFC-12, HFC-134a, HFC-134 and HFC-12 5 have been investigated, and employment of HFC-134a has been expected.
As a lubricating oil for a refrigerator of using a refrigerant HFC-134a, synthetic oxygen-containing hydrocarbon oils, such as esters and polyesters, which are compatible with the refrigerant, have heretofore been investigated. In particular, since synthetic ester base stocks have a better electric insulating property and a better high-temperature compatibility with the refrigerant and have other excellent characteristics of a lower hygroscopic property than do synthetic polyether oils, the synthetic ester base stocks are expected to be useful.
Use of synthetic ester base stock as a refrigerator oil is disclosed, for example, in JP-A-56-133241 and JP-A-59-164393 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); refrigerator oils of synthetic ester base stocks for specific refrigerants of chlorofluorohydrocarbons and fluorohydrocarbons are mentioned, for example, in JP-A-2-276894; and refrigerator oils of synthetic ester base stocks for specific refrigerants of hydrogen-containing fluorocarbons are mentioned, for example, in JP-A-3-88892, JP-A-3-128991 and JP-A-3-128992. Refrigerator oils composed of a synthetic ester base stock and a phosphate or phosphite are disclosed, for example, in JP-A-55-92799, JP-A-56-36570, JP-A-56-125494, JP-A-62-156198 and JP-A-3-24197; and heat pump oils composed of them are mentioned, for example, in JP-B-57-43593 (the term "JP-B" as used herein means an "examined Japanese patent publication"). In addition, refrigerator oils containing a thiophosphite, an epoxy compound and a methanesulfonate are disclosed in, for example, JP-A-56-36569, JP-A-58-15592 and JP-A-62-292895.
However, since synthetic ester base stocks are chemically more active than hydrocarbon oils, they easily form particular sludge in a compressor to cause a higher temperature. In addition, the ester oils do not have good wear resistance. Hitherto, improvement in the wear resistance and thermal stability of the ester oils by adding certain additives thereto has been investigated. However, it is still not satisfactory for preventing sludge formation and wear specific to ester oils.